Research

Key Parameters in Common Envelope Evolution

Binary systems can be comprised of any combination of astrophysical
objects, from stars and planets to neutron stars and black holes. In
a post-LIGO-detection age, we endeavor to explain the existence of
close binaries that are able to merge on a timescale less than the
age of the universe, as well as the rate of such mergers. Though the
start-to-finish evolution of these systems in general is of
interest, it is practical to investigate phases of evolution that
offer the possibility of being observed; the traditional formation
channel for close binaries, in which a common envelope (CE) phase
dramatically reduces the separation of a pre-existing binary system,
offers that possibility.

The onset of the CE phase occurs when the expanding envelope of one
of the stars in the binary extends to the orbit of the other star
(or planet, black hole, etc.) and engulfs it (a → b). Due to
gravitational interactions between the embedded object and the
envelope material, a drag force acts on the former and it eventually
plunges in toward the core of the expanding star (b). Depending upon
the amount of energy the embedded object can deposit into the
envelope material and the timescales on which this energy can be
transported throughout the envelope, the CE phase may end in either
the ejection of the envelope and a surviving binary comprised of the
embedded object and the remaining core at greatly reduced separation
(c), or a merger of the two with envelope intact (d).

My work focuses on key physical parameters that describe the CE
phase for a range of initial systems, giving insight on the inspiral
phase and the final system configuration. This includes looking at
the impact of envelope structure on drag forces and expected
accretion, as well as inspiral and energy transport timescales.

Common Envelope Evolution and LIGO Source PopulationsRosa Wallace Everson, Morgan MacLeod,
Soumi De, & Enrico Ramirez-Ruiz
ABSTRACT: With confirmed gravitational wave detections of binary
neutron star (BNS) and binary black hole (BBH) mergers, the
channel through which both populations form remains an open
question. Common envelope (CE) evolution plays a role in shaping
these populations as one of the few formation channels in which
the separation of a field binary may be reduced such that the
resulting stellar remnants can merge in a Hubble time. CE
evolution may include several different inspiral stages from
onset to completion, including a quick dynamical phase and a
gradual self-regulated phase of orbital decay, the length and
characteristics of which impact whether the binary will merge
during CE, become a gravitational wave source progenitor, or
remain at wide separation. Recent work has shown that CE
evolution depends upon more than initial conditions: the
structure of the envelope impacts the duration of inspiral and
the post-CE properties of the embedded compact object. We
explore the implications of including envelope structure in both
the BNS and BBH progenitor cases, with new considerations for
how the dynamical phase of CE inspiral should be approached, and
how these affect the types of systems that we will observe in
the future with LIGO.

Effects of the Common Envelope Phase on Binary Black Hole
EvolutionRosa Wallace Everson, Phillip Macias,
Morgan MacLeod, Andrea Antoni, & Enrico Ramirez-Ruiz
ABSTRACT: The detection of gravitational wave signals from
binary black hole (BBH) mergers in recent years has raised
pressing questions about the formation and characteristics of
these systems. In order for BBHs produced in the traditional
formation channel to merge in a Hubble time, the pair must
undergo a common envelope (CE) phase to dramatically reduce the
separation distance of the progenitors prior to CE ejection.
Recent work on the CE phase has shown that density gradients in
the envelope material produce a significant departure from drag
and accretion rates of the embedded compact object as predicted
by Hoyle- Lyttleton accretion (HLA) formalism; these effects, in
turn, have implications for mass and angular momentum transfer
between the donor star and compact object. Using a range of
simplified progenitor systems in which a massive, stellar-mass
black hole (BH) dynamically inspirals through the envelope of a
giant stellar companion, we examine these CE effects.

Undergraduate Research

An Observationally Constrained 3D Potential-field
Source-surface Model for the Evolution of Longitude-dependent
Coronal StructuresRosa Wallace Everson & Mausumi
Dikpati
An exploration of using morphological data to constrain models
of the magnetic field in the solar corona